Optical position encoders determine the displacement of a readhead relative to a scale that includes a pattern that is detected by the readhead. Typically, position encoders employ a scale that includes at least one scale track that has a periodic pattern, and the signals arising from that scale track are periodic as a function of displacement or position of the readhead along the scale track. Absolute type position encoders may use multiple scale tracks to provide a unique combination of signals at each position along an absolute scale.
In various applications, an optical encoder readhead will comprise multiple metal layers within detector electronics. In some embodiments, the metal layers may be used to block light which may affect circuitry components such as transistors, diodes or other components which may be undesirably affected by stray light which will result in undesirable signal corruption. It is desirable in various applications for a readhead to be as compact as possible and for the detector electronics of such a readhead to comprise a monolithic detector configuration within a single integrated circuit. As a readhead becomes more compact, the possibility of stray light resulting in corrupted signals becomes stronger, especially as metal layers have shorter overlap distances. Stray light is a larger problem with light that is not constant, e.g., light which is reflected or transmitted by a scale which comprises a periodic pattern varying according to displacement of the readhead along the scale track. It is therefore desirable to avoid light transmission along a layer between metal layers to components which are not intended to sense light which may result in signal corruption.